(a) Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and, more particularly, to an improvement of process control for manufacturing a semiconductor device.
(b) Description of the Related Art
Si-epitaxial layer (or hereafter, simply referred to as epitaxial layer,) is widely used as a collector region for a bipolar LSI family. Conventionally, an infrared interference is used for the measurement of the thickness of the epitaxial layer.
The infrared interference uses the property that the refractive index of Si changes considerably when exposed to infrared radiation depending on the level of impurity density in a Si substrate. FIG. 1 shows an example of a conventional measurement in a sample wherein an epitaxial layer of a non-doped or lightly doped layer is grown after doping of antimony in Si substrate in a density of more than 1.times.10.sup.18 /cm.sup.3. The thickness is measured by an interference between a reflection from the epitaxial surface 18 and a reflection from the interface between the epitaxial layer 18 and the Si wafer 11 (or epitaxial/wafer interface).
The measurement of thickness by the infrared interference method enables measuring a wide range of thickness of Si epitaxial layer covering from about a few-.mu.m to some-10 .mu.m without destroying the Si epitaxial layer. It is widely known as an "Epitaxial Layer Thickness Control Method", as described in, for example, "Semiconductor Crystal Material General Handbook" issued by K.K. Fuji Techno-system Page 574-575 issued Jun., 20, 1986
The infrared interference method for measuring the thickness of an epitaxial layer has a deficiency that the method cannot measure an ultra thin layer having a thickness less than 0.5 .mu.m. This is because the interference of reflected waves does not occur when the wavelength of the infrared ray is longer than the epitaxial layer thickness. In this respect, the measurement of an oxide film formed on the Si surface in thickness less than 0.1 .mu.m can be measured by using the interference of visible light which has a shorter wavelength than that of infrared ray. However, in the case of a Si epitaxial layer, the difference of refractive index within visible light is very little and the reflections do not cause any measurable interference.
In view of the recent trend for higher integration and performance of the LSI, a thinner epitaxial layer is desired which necessitates controlling the epitaxial layer thickness on the order of less than 0.5 .mu.m. For example, in the development of a next generation MOS transistor having a gate width of 0.1 .mu.m, the use of the epitaxial layer for the source/drain regions and a channel region is proposed. The thickness of the epitaxial layer will be less than 0.1 .mu.m in this case.
Another method using a destructive measurement is known, as described in Patent Publication No.JP-A-2(1990)-105438. In this method, a SiO.sub.2 film 29 is patterned to have openings of a few square millimeter on the silicon wafer 21 as shown in FIG. 2A, then an epitaxial layer 28 is selectively grown in the openings, as shown in FIG. 2B. Thereafter, the wafer is dipped in a dilute HF (hydrofluoric) solution for removing the SiO.sub.2 oxide pattern 29 shown in FIG. 2C. The distance between the Si surface 21 and the Si epitaxial layer can be measured by using a bandgap meter to obtain the thickness of the Si epitaxial layer.
The destructive method has also a problem, in that it is applicable only in the special occasion wherein the openings provided for exposing the Si surface in an oxide layer is filled with an epitaxial layer selectively grown in the openings. Moreover, since it takes expertise and a longer time for measurement, a test wafer is needed in addition to the regular products, which increases total manufacturing cost. As a result, a new measurement is desired for measuring an ultra-thin Si epitaxial layer without the use of the infrared interference widely used currently.